144 Experimental Philosophy. [Lecture 10. 



ing from a point, continue to recede from each 

 other, and exhibit the form of an inverted cone. 



8. A small object, or a small single point of 

 an object, from which rays of light diverge or 

 indeed proceed in any direction, is sometimes 

 called the radiant, or radiant point. 



9. Any parcel of rays, diverging from a point, 

 considered as separate from the rest, is called a 

 pencil of rays. 



10. r Fhe focus of rays is that point to which 

 converging rays tend, and in which they unite 

 and intersect or cross each other. It may be 

 considered as the apex or point of the cone ; and 

 it is called the focus (or fire-place), because it is 

 the point at which burning-glasses burn most 

 intensely. 



11. The virtual or imaginary focus is that 

 supposed point behind a mirror or looking-glass, 

 where the rays would have naturally united, had 

 they not been intercepted by the mirror. 



12. Plane mirrors or speculum? are those re- 

 flecting bodies, the surfaces of which are per- 

 fectly plain or even, such as our common look- 

 ing-glasses. Convex and concave mirrors are 

 those the surfaces of which are curved. 



13. An incident ray is that which comes from 

 any body to the reflecting surface ; the reflected 

 ray is that which is sent back or reflected. 



14. The angle of incidence is the angle which 

 is formed by the line which the incident ray 



